feat!: probes link to the panel; signed release updates for container nodes [CI]

Probes connect to the panel directly and get the agent list as desired state.
CI signs the image digest of agent and probe releases, the panel forwards the
manifest, and a host updater verifies it with a local public key, pulls by
digest and rolls back when the new version is unhealthy.

backend/tests/Monitor.Backend.Tests/OperationsAndSecurityTests.cs

@@ -175,9 +175,8 @@ public sealed class OperationsAndSecurityTests
             Assert.Equal("fleet_unsupported", reply.Error!.Data!.Code);
         }
 
-        // 连接仍然可用
-        ProbeAgentsResult list = await agent.RequestProbeAgentsAsync();
-        Assert.Contains(list.Agents, a => a.AgentId == agent.NodeId);
+        // 连接仍然可用：路由器照常回复
+        Assert.Equal("fleet_unsupported", (await agent.RequestRawAsync("fleet.exec")).Error!.Data!.Code);
     }
 
     [Fact]
@@ -195,9 +194,6 @@ public sealed class OperationsAndSecurityTests
     public async Task PendingNodes_CannotCallProductMethods()
     {
         await using AgentSimulator agent = await _backend.ConnectAgentAsync(autoApprove: false);
-        RpcMessage reply = await agent.RequestRawAsync(MonitorMethods.ProbeAgents, JsonSerializer.SerializeToElement(new ProbeAgentsParams(), MonitorJsonContext.Default.ProbeAgentsParams));
-        Assert.Equal("fleet_node_pending", reply.Error!.Data!.Code);
-
         // 状态与事件也不会被接受：监控数据里没有它
         await agent.SendEventAsync(AgentEventKinds.ProbeConnected, _backend.DefaultProbeId);
         await Task.Delay(200);